Dry soils in Mexico trigger extreme ‘hot droughts’ across the Southwest

A new study has pinpointed an upstream trigger for the Southwest’s most punishing heat-and-dryness spells. When soils across northern Mexico dry out, “hot droughts” are more likely to flare hundreds of miles downwind in Arizona, New Mexico, and Texas – and to persist through the night.

The work, published in the journal Geophysical Research Letters, argues for treating hot droughts with the same urgency as hurricanes: with forecasts, alerts, and clear public guidance.

When heat meets drought

The researchers define a hot drought as a period when at least two weeks of abnormally low rainfall overlap with at least three straight days of unusually high temperatures.

Heat and drought feed on each other. Parched ground sheds less moisture to the air. With little evaporation to soak up energy, more of the sun’s power goes into heating the surface and the air above it. Crops suffer. Wildfire risk jumps. People working or recreating outdoors face greater danger.

“Hot droughts will propagate to other parts of the country and have detrimental effects on health, on infrastructure, on daily life,” said senior author Enrique Vivoni, a hydrologist at Arizona State University.

“We need systems to alert us to hot drought just like we have systems that alert us to hurricanes.”

Record-breaking heat of 2023

The team focused on the summer of 2023 – an outlier even by desert standards. Typical afternoon highs in many parts of the region run from 95 to 104°F (35 to 40°C). During the hot drought window, temperatures spiked up to 14°F (8°C) above those baselines.

The North American monsoon was weak. Weather patterns throttled the normal flow of moisture from the Pacific, leaving the Southwest and northern Mexico with little relief.

Using weather station data, rain gauges, and a soil moisture dataset that blends satellite and ground observations, the authors put 2023 in a 40-year frame. By their metric, the event reached nearly five times the severity of the region’s average hot droughts since 1980.

“Lack of rainfall can increase heat, which can further intensify the loss of water,” said co-author Somnath Mondal, a hydroclimatologist at Northeastern University. “It’s a reinforcing loop.”

Heat born in Mexico

One of the study’s clearest signals came from across the border. In healthy monsoon years, rainfall over northern Mexico evaporates back into the atmosphere, helping recharge clouds downwind in the United States.

In 2023, that land-atmosphere “recycling” faltered. The soils in northern Mexico were too dry to contribute much moisture back to the air. The result, the authors argue, was a supply-side shock that helped set the stage for extreme heat and dryness in the U.S. Southwest.

“In 2023, Mexico influenced Arizona’s hot drought in a stronger way than the soil of Arizona itself,” Mondal said. He and Vivoni also found that this cross-border linkage has strengthened since 1980. Dry winds and depleted moisture, it turns out, do not respect political lines.

“We know we receive water vapor, clouds, and rain from Mexico,” Vivoni said. “We didn’t know we could also receive a hot drought.”

When nights stop cooling down

The second surprise was how little relief came after dark. In deserts, nights usually offer a reset. Heat stored in rocks and soil radiates back to space, and temperatures drop fast.

But not in 2023. The daytime heat load was so extreme that it did not fully dissipate overnight. Residual warmth lingered in the boundary layer. Each hot day fed the next hot night, which then primed the following day – a compounding cycle that lasted for weeks.

That pattern has grown more common over the past four decades, the authors found. It showed up even in rural areas, which typically cool faster than cities after sunset.

The health stakes are obvious. When nights stay hot, bodies cannot recover. Risks of heat illness and death rise, even for people who wake early to work or hike before sunrise.

“There isn’t a good understanding that in a hot drought, you need to take more precaution than if it’s just a heat wave,” Vivoni said.

From findings to drought forecasts

The authors say the findings should translate into action. Monitoring upwind soil moisture could provide an early warning for downwind communities. If northern Mexico dries out, the Southwest may be next.

Public agencies could use that signal to limit outdoor work hours and pre-position cooling centers. Moreover, they should adjust firefighting readiness, and prepare hospitals for heat-related cases. Utilities could brace for surging demand as air conditioning use climbs.

Hot drought alerts would not replace heat advisories or drought monitors. They would knit them together. The point is to flag the compound risk, and the added danger when nights fail to cool down.

Modeling future hot droughts

The current study uses observations to map the pathway from dry soils to hot drought downwind. The next step is mechanistic.

Vivoni and Mondal plan to build physics-based models that track how these events propagate through the atmosphere. They also want to test whether similar teleconnections exist in other monsoon-influenced regions, such as along the India–Pakistan border.

“As climate change continues, more places will likely experience the dry soil conditions that spur and spread hot droughts,” the authors wrote. The 2023 event shows how quickly those conditions can cascade.

“Climate doesn’t respect national borders. We’re more interconnected than we thought,” Vivoni concluded.

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